Cathode ray tube apparatus



Jan. 21, 1958 Filed April l5 1948' Itwenfors:

James Dw-yeY McGee Hans Ge'rh-ard Lubazyns/(i Reginald Sidne Weble'] By/= W L Afforn e1;

hired fitates CATHODE RAY TUBE APPARATUS 3 Claims. (Cl. 315-9) This invention relates to cathode ray tube apparatus.

A difliculty frequently encountered in using cathode ray tube apparatus is that the display or presentation on the fluorescent screen of the cathode ray tube does not persist for a sufliciently long time. For example, in the case of the navigation apparatus known as the Plan Position Indicator, in which a presentation is produced periodically on the screen of a cathode ray tube in response to signals picked up by a rotating aerial, if the aerial rotates relatively slowly, for example once every 30 seconds, the presentation will fade quickly and possibly disappear altogether during each rotation of the aerial. Even if the aerial rotates relatively rapidly the repetition frequency of the signal is generally so low that the fading produces an undesirable flickering of the presentation. The fading can be reduced to some extent by employing a long-persistence phosphor in the screen of the tube but long-persistence phosphors have certain disadvantages, one of which is that in general they are of low luminosity.

Thus the present invention is concerned with cathode ray tube apparatus of the type adapted to display electrical signals on the screen of a cathode ray tube, said screen having a decay time which is so short in relation to the desired display time for said signals as to give rise to objectionable variations in brightness such that the visibility of said display is impaired.

The object of the invention is to provide means for improving the visibility of electrical signals reproduced by a cathode ray tube.

According to one feature of the invention, cathode ray tube apparatus comprises means for producing a charge image of electric signals in a storage device, and means for scanning said charge image a plurality of times so as to derive signals therefrom, and means for reproducing said derived signals on the screen of a cathode ray tube a plurality of times.

In order that the invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the drawing in which:

Figure 1 is a diagrammatic view of apparatus according to one embodiment of the invention, and

Figure 2 is a diagrammatic view of apparatus according to another embodiment of the invention.

Referring to Figure 1, which illustrates apparatus suitable for use in a Plan Position Indicator, there is shown an electron discharge tube 1 having an envelope which contains a target 2 of the double-sided mosaic type with which is associated a signal plate 3 in the form of a grid or mesh. At one end of the tube is provided an electron gun 4 and at the other end of the tube is provided an electron gun 5. The electron beam from gun 4 is arranged to execute a scanning raster, whereby it scans target 2, under the action of deflecting coils 6, and the beam is arranged to be modulated in accordance with signals received by an aerial indicated at 7, and applied to the control electrode of gun 4 through an electronic switch 8. The electron beam from gun 4 is of relatively atent O T 2,820,921 Patented Jan. 21, 1958 high velocity and releases secondary electrons at target 2, these secondary electrons being collected by signal plate 3. Thus a positive charge image corresponding to the received signals is built up on target 2. Electron gun 5 is arranged to provide a low-velocity beam which scans target 2, under the control of deflecting coils 9, so as to restore target 2 to the potential of the cathode of gun 5, and the return beam from target 2 is applied to a collector formed by an electron multiplier 10, lift plates 11 being provided for separating the forward and return beams in the well known manner. The beams from electron guns 4 and 5 are focussed by means of coil 12, and accelerating potentials for the beams are provided by electrodes 13 and 14 respectively, which may consist of metallic coatings on the inner surface of envelope 1.

The output from electron multiplier 10 is applied to an amplifier 15. A part of the output of amplifier 15 is applied to the control electrode of a cathode ray tube 16 which has a short decay time, the beam of which is arranged to be deflected by means of deflecting coils 17 whereby the beam is caused to scan the screen of the tube and an image, corresponding to the original signals applied to electronic switch 8, is formed on the screen of tube 16. Another part of the output from amplifier 15 is returned by lead 15a to electronic switch 8 and is fed through the switch to gun 4, whereby the charge image on target 2 is renewed.

Assuming now that cathode ray tube 16 forms part of a Plan Position Indicator and that aerial 7 is a rotating aerial, then any signals picked up by the aerial during each rotation, which signals would normally be employed directly for producing a presentation on the screen of tube 16, are fed through switch 8 to electron gun 4 so that a charge image is built up on target 2. After one rotation of the aerial, the switch 8 operates under the action of any suitable means to disconnect the aerial 7 from the gun 4 and during the next rotation, or if desired a longer period, the charge image is scanned by the beam from the gun 5 and the signals thereby derived are applied to the tube 16 and reproduced on the fluorescent screen thereof. The beam from the gun 5 discharges each element of the mosaic and thus tends to discharge the charge image during each scan, but an amplified form of the derived signals is fed from the amplifier 15 to the gun 4 so as to renew the charge image as it is discharged. The beam from gun 4 may be arranged to lag a few lines behind the beam from gun 5 so that as the beam from gun 5 scans one side of target 2, and reduces the charge image thereon, the beam from gun 4 follows it and restores the charge image. This procedure results in the charge image slowly moving across the target 2 but this is not disadvantageous if the periods during which the target 2 is scanned by the beam from the gun 5 are not unduly long. This scanning of target 2 may be repeated at any desired rate and with a suitably high repetition rate the presentation on the screen of tube 16 may be arranged to appear continuous and substantially free from fading, and the repeated scanning of target 2 may be continued substantially throughout the period of rotation of aerial 7 so that the presentation persists until the aerial is again connected to the gun 4 and a further charge image is formed. The electronic switch 8 may be arranged to disconnect amplifier 15 from gun 4 shortly before the aerial is again connected to the gun 4 so that the charge image produced on target 2 by one train of signals from the aerial is removed by the beam from the gun 5 in readiness for the building up of a further charge image when a further train of signals is received from the aerial.

In a modification of this embodiment of the invention electronic switch 8 may be arranged to feed the signals from aerial 7 directly to cathode ray tube 16 as well as to electron gun 4, so that a presentation is produced directly by the received signals and is followed by similar presentations resulting from the signals derived from target 2.

Referringto Figure 2, the apparatus comprises an e1ectron discharge tube 18 having an envelope containing an electron gun 19 and a target in the form of a-photoelectric mosaic 20'which is carried by a transparent glass or mica plate 21 and with which is associated a signal plate 22 in the form of a grid or mesh. Mosaic 20 may be formed on plate 21 by evaporation of a suitable substance through signal plate 22. Gun 19 is arranged to produce a low-velocity electron beam which is focussed by coil 23 and is deflected by means of deflecting coils 24 so as to scan mosaic 2t). Signal plate 22 is maintained at a potential which is slightly more positive with respect to the cathode of gun 19 than the potential corresponding to the first cross-over point on the secondary emission characteristic of the photoelectric mosaic 20. The potential of signal plate 22 may, for example, be volts. A cathode ray tube 25 is arranged with its fluorescent screen facing mosaic and the electron beam of said tube is arranged to execute a scanning raster so that it scans the screen of the tube which tube has a short decay time. Signals received by a rotating aerial 26 are fed to tube 25 so that an optical image is formed on the screen of the tube corresponding to the received signals. An optical system, represented by the lens 27, is provided for projecting said image on to mosaic 20, whereby a charge image is formed on mosaic 20. This charge image may have a persistence of several hours. It does not comprise gradation, that is to say, the charges produced at various areas of mosaic 20 correspond either to full white or full black. It is believed that the charge image is generated owing to the illuminated elements of the mosaic becoming positively charged, due to the loss of photo-electrons, beyond the first cross-over point of the above-mentioned characteristic. The scanning of the mosaic 20 by the beam from the gun 19 causes the emission of secondary electrons from the elements of the mosaic and said electrons proceed to the signal plate 22 whereby signals are generated at the latter. The beam is unable to reduce the elements to cathode potential due to said elements being above the said cross-over point. The signals resulting from the scanning of the charge image by the electron beam from gun 19 are fed to an amplifier 28 and the output of amplifier 28 is applied to the control electrode of a cathode ray tube 29 so as to provide on the screen of tube 29 a presentation corresponding to the signals originally received by aerial 26. The tube 29 has a short decay time.

By repeated scanning of the charge image a series of similar presentations can be'produced on the screen of cathode ray tube '29, and if the scanning "repetition rate is suitably chosen the said presentations can be arranged to appear as a continuous prolonged presentation substantially free from fading and flicker. When it is desired to remove the charge image in readiness for the production of a further prolonged presentation this may be eflected by applying a negative pulse to signal plate 22. During the occurrence of this pulse no secondary electrons can escape from any elements of the mosaic screen and thus the whole of the mosaic is brought down to cathode potential due to the elements being discharged by the electron beam. If desired, means may be provided for automatically applying a negative pulse to the signal plate at an appropriate time during each rotation of aerial 26.

We claim:

1. A circuit "arrangementicomprising an electronic charge storage device having a storage target therein, means for developing a first electron beam, means for modulating said beam in response to electrical signals, first deflecting means for deflecting the developed beam to cause it to execute a scanning raster on said target to produce a charge image on said target in accordance with said electrical signals, means for developing a second electron beam, second means for deflecting the developed second electron beam to cause it to scan said charge image a plurality of times at a frequency to produce derived signals which are repetitions of said electrical signals, a signal reproducing tube having a screen adapted to display said electrical signals and having a decay time short in relation to a desired display time for said electrical signals, means including an amplifier for applying said derived signals to said signal reproducing tube to eproduce said repetitions to provide said desired display time, and means for feeding said derived signals via said amplifier to said modulating means to modulate said first beam, whereby said charge image on said target as it is reduced by said second electron beam is renewed by the signals fed via said amplifier.

2. A circuit arrangement comprising a cathode ray tube, means for feeding electrical signals to said tube, said tube having a fluorescent screen adapted to display said signals and having a decay time short in relation to a desired display time for said signals, an electronic storage device .having a charge storage target therein, means for generating a first electron beam, means for modulating said beam by said signals, means for scanning said beam over said target to form a charge image thereon correlative with said signals, means for generating a second electron beam, means for scanning said second beam over said target to discharge said charge image and to derive signals which are repetitions of said electrical signals, means for amplifying said derived signals and means for applying the amplified signals to said target to renew said charge image, said last-mentioned scanning means being operative to scan said renewed charge image to obtain a further repetition of said electrical signals and means for feeding said repetitions of said electrical signals to said cathode ray tube to reproduce said repetitions, whereby the visibility of said display is improved.

3. A circuit arrangement comprising an electronic charge storage tube having a target electrode, means for applying signals to said target electrode thereby to form a charge image thereon, means for generating a beam of electrons, means for scanning said target with said beam of electrons a plurality of times to generate a plurality of sequences of similar signals representative of the signals applied to said target electrode, means for feeding part of said generated signals back to said target electrode to etfect a renewal of said charges on said target, a cathode ray tube having a luminescent screen, said screen having a decay time which is shortcompared with the time required for a display thereon, and means for applying said plurality of sequences of signals to said cathode ray tube thereby to form a display having a required display time.

References Cited in the file of this patent UNITED STATES PATENTS 2,213,174 Rose Aug. 27, 1940 2,213,178 Iams Aug. 27, 1940 2,412,669 Bedford Dec. 17, 1946 2,429,809 Fyler Oct. 28, 1947 2,500,633 Edwards Mar. 14, 1950 2,549,072 Epstein Apr. 17, 1951 2,602,921 Peters et al. July 8, 1952 2,667,635 Hergenrother Jan. 26, 1954 

